The invention relates to image processing for suppressing illegal reproduction of a secret document or the like.
Recently, electrophotographic copying machines are used widely, and anyone can copy a document including characters and/or images printed on a sheet of paper or the like easily. Especially, in color digital copying machines and multifunctional peripherals available recently, technologies on image processing and on image forming are improved very much, so that a reproduction very hard to be discriminated from an original can be produced easily. Therefore, it is an important problem to prevent illegal copying of a secret document or the like.
In order to prevent illegal copying and leakage of a print, on which a reproduction is inhibited, it is known to embed an additional image which cannot be recognized or is hard to be recognized with naked eye in the print. The additional image added to a document becomes visualized and can be observed clearly in a print thereof after it is printed with an image forming apparatus such as a printer (refer to, for example, Japanese Patent laid open Publications 2001-197297, 2004-201068 and 2004-228897). The additional image is intended to suppress illegal copying thereof psychologically. For example, if a sentence such as “COPY INHIBIT” is embedded as the additional image in an image of an original, when it is copied, a reproduction thereof is not similar to the original, or the sentence embedded in the image of the original is visualized in the reproduction. Therefore, when one sees the reproduction, it is clear that it is not the original because the sentence such as “COPY INHIBIT” which cannot be recognized or is hard to be recognized with naked eye is visualized in the original.
An example for adding information to be recognized with naked eye to a document is described in, for example, Japanese Patent laid open Publication 2004-153568. A document image is liable to become yellow, dirty or the like due to deterioration in various factors. On the other hand, a color original document image may become necessary when a large number of copies are produced with a monochromatic copying machine are printed and distributed. Then, in a copying machine described in Japanese Patent laid open Publication 2004-153568, information on the original document such as a barcode or uniform resource locator (URL) for representing a storage position of the original document is added to an image to be printed. Then when the printed image is put and scanned on a platen in the copying machine, the original image can be taken out automatically. Further, when a document is read and the information on the document is recognized, the portion for the information can be printed black in order to prevent unnecessary exposure of the information on the position of the original and to improve the security of the original. It is to be noted that this technology is not related to the prevention of illegal copying.
When an original with an additional image to suppress illegal copying as mentioned above is copied in an image forming apparatus, even when a creator of the original copies it, an image of the reproduction clearly shows that it is not an original. Thus he or she cannot get a print which is the same as a document wherein the additional image is not visualized or a document which is the same as an original without the additional image. However, it is desirable that a legitimate user such as a creator of the document can produce a copy which is the same as the document or the original. It is also to be noted that a situation wherein a copy which is the same as the document or the original is needed happens suddenly in usual cases. Then, a copying machine which is needed to be connected to a network, as described in Japanese Patent laid open Publication 2004-153568, cannot deal it.